His mum Hayley, 23, and dad Frank, 41, first noticed something was wrong when Marshall tried to take his first steps.

"His feet kept on turning outwards when he tried to walk. He would be very clumsy. We took him to our doctor to get him checked out, but we would never have imagined that he could have arthritis," said Hayley.

"I only thought it was something you got if you were elderly. I was absolutely gobsmacked. I still remember the words - it was quite a shock. I was worrying that my other children could have it as well, but they said it was very unlikely two siblings will have it."

Doctors have been unable to identify why Marshall has developed the condition. He has the second most common type of juvenile idiopathic arthritis which affects between 10,000 and 15,000 children in the UK.

Hayley, who is three months pregnant and also mum to twins Frankie and Sommer, four, and Caitlyn, seven months, said she struggled to accept the idea of her little boy having chemotherapy when Marshall was first diagnosed, as side-effects of the treatment can include liver failure, kidney failure and blood cancer.

"Marshall can walk. On good days he's still very wobbly and he falls over quite a lot. Then, on bad days, he can't move. He screams for hours. He'll just push himself against a wall to try to get up, or bum-shuffle around the floor.

"About three or four months after he was diagnosed he literally could not move, he couldn't stand and he was that way constantly, every day. His brother and sister would go and play in the back garden and he wouldn't go out with them. When we have good days now, it is a relief.

"The other kids are brilliant with him, they play with him. He wants to do, and he does try to do, all the things that Frankie and Sommer will do but he can't always do them. But he still gives it a go.

"To see him smile is really nice, having seen him in so much pain. It's like he is showing me that I'm doing something right to try to control his condition.

"It was a big decision for me to actually accept his chemo treatment every week, because of the side effects it can have. I panicked, thinking 'I don't want him to have more problems'. Then I thought 'I've got to, otherwise it's not helping him'. I think it's the best thing I've done.

"You don't know what each day holds. One day he could get up and be running around, the next day he could not move. He can't talk so it's difficult for him to let us know how he is feeling.

"Sometimes he might scream in pain, but he's always still got a smile on his face."